1. Field of the Invention
The present invention relates to a Wireless Sensor Network (WSN) and problems associated unstable link states. More particularly, the present invention relates to the technology of transmitting/receiving data in a WSN.
2. Description of the Related Art
A WSN network differs from the existing conventional networks that have been realized for communication for at least the reason that the WSN has been embodied for the purpose of collecting remote information. The WSN is equipped with a sensor node for processing information collected via a sensor and then transfers processed information, A sink node is used for sending transferred information to outside the network. As a network is constructed of a large number of sensor nodes, the structure of each sensor node should be simply designed. Also, since a certain sensor node may be arranged in an area in which it is difficult for a person to gain access, the sensor nodes should be designed to consume a relatively small amount of electric power so that the sensor node may operate for up to several months or several years from an initial battery without requiring replacement. In addition, the sensor nodes should be designed to have mobility so that each position in which the sensor node has been installed is enabled for free movement. Furthermore, even though some sensor nodes existing within the network might become damaged, the WSN must be embodied so as not to affect the maintenance of the network.
Meanwhile, IEEE 802.15 Working Group defines the standards for a short-distance wireless network, and in particular, since IEEE 802.15.4 standards defined by the IEEE 802.15 Working Group enables a low-power short-distance wireless network to be commercially realized, the IEEE 802.15.4 standard is raising its head as core technology which is suitable for being applied to a sensor network.
Further, the WSN based on the IEEE 802.15.4 standard protocol includes proposed methods for transmitting data, respectively corresponding to three different cases.
FIG. 1 is a conceptual view illustrating the network structure of a WSN based on the IEEE 802.15.4 standard protocol according to the prior art, and FIGS. 2, 3, and 4 are flowcharts illustrating the respective processes of a method for transmitting data according to the prior art.
To begin, in FIG. 1, the WSN based on the IEEE 802.15.4 standard protocol is constructed in a network structure having a multi cluster form in which a star topology and a peer-to-peer topology are combined. Specifically, the WSN includes: a first cluster CID1 (100) realized with a Personal Area Network (PAN) coordinator as the center; a second cluster CID2 (200) which includes a first cluster hub CLH1 linked to a device included in the first cluster CID1 (100), and is realized with the first cluster hub CLH1 as the center; a third cluster CID3 (300) which includes a second cluster hub CLH2 linked to a device included in the second cluster CID2 (200), and is realized with the second cluster hub CLH2 as the center; and a fourth cluster CID4 (400) which includes a third cluster hub CLH3 linked to the second cluster CID2 (200), and is realized with the third cluster hub CLH3 as the center.
The aforementioned multi-cluster formation includes the PAN coordinator as the center. In the first place, if the PAN coordinator forms the first cluster CID1 by performing functions, including network settings, beacon transfer, node management, node information storage, and message route setting between connected nodes, devices included in the first cluster CID1 100 may scan a specified channel list so as to check for a usable communication channel. Then, if a Wireless Personal Area Network (WPAN) IDentification (ID), which is not duplicate, is selected following the completion of scanning, a Full Function Device (FFD), which can directly transmit/receive data, functions as the first cluster hub CLH1. Thereafter, if the first cluster hub CLH1 transmits a beacon frame to other devices, the devices all of which receive the beacon frame are linked to the first cluster hub CLH1, and then form the second cluster CID2 200. With the repetition of this process, the third and the fourth clusters CID3 300 and CID4 400 are embodied, and finally, one WSN is formed.
Still referring to FIG. 1, if a device 10 of the first cluster CID1 100 transmits data up to a device 3 of the second cluster CID2 200 within the WSN formed as described above, the WSN sets a route for data transfer in the first place. For example, a data transfer route can be set in a sequence, such as CID1-10→CID1-3→CID1-0→CID1-6→CID2-0→CID2-1→CID2-3. Namely, in FIGS. 2, 3, and 4, an apparatus for transmitting/receiving data (i.e., a data transmit/receive apparatus) of a departure location may be the device 10 of the first cluster CID1 illustrated in FIG. 1, and in FIGS. 2, 3, and 4, a data transmit/receive apparatus existing in a first transit node to a n-th transit node can be either the device 3 of the first cluster CID1, the device 0 of the first cluster CID1, the device 6 of the first cluster CID1, the device 0 of the second cluster CID2, or the device 1 of the second cluster CID2, illustrated in FIG. 1. In FIGS. 2, 3, and 4, a data transmit/receive apparatus of a destination can be the device 3 of the second cluster CID2 200 illustrated in FIG. 1.
Hereinafter, with reference to a configuration of the WSN exemplified as above and flowcharts depicted in FIGS. 2, 3 and 4, a description will be made in detail of a transfer process relevant to each case.
Case 1. A Successful Transfer of a Data Frame and Receipt of an ACKnowledgement (ACK) Frame:
With reference to FIG. 2, after a transfer route of a data frame is set, the data transmit/receive apparatus of the departure location transmits the data frame to a data transmit/receive apparatus located in the first transit node (step S11), and then waits for a transfer ACKnowledgement (ACK) for a predetermined waiting time (step S20). Before receiving such an acknowledgement, the data transmit/receive apparatus located in the first transit node transmits a data frame to a data transmit/receive apparatus located in the second transit node (step S12). Upon receipt of the data frame, the data transmit/receive apparatus located in the second transit node delivers a data frame to a data transmit/receive apparatus located in the next mode on the transfer route. Subsequently, data transmit/receive apparatuses located in different nodes within the transfer route repeat the above processes, and finally, deliver a data frame to a data transmit/receive apparatus of the destination apparatus (steps S13, S14, and S15). The data transmit/receive apparatus of the destination generates an ACK frame in reply to the reception of data, and then transmits the generated ACK frame (step S31). Thereafter, the data transmit/receive apparatuses located on the transfer route set during the data transfer deliver the ACK frame to the data transmit/receive apparatuses of the departure location (steps S32, S33, S34, and S35). Finally, if the data transmit/receive apparatus of the departure location receives the ACK frame (from the destination apparatus via the transit nodes) within a predetermined waiting time for which it waits for a transfer ACK, the apparatus at the departure location confirms that data has been successfully transmitted to the destination apparatus (step S40).
Case 2. An Unsuccessful Transfer of a Data Frame
With reference to FIG. 3, in the case of unsuccessful transfer of a data frame, by performing steps S11 to S13 as in the case described above (i.e., in the case of successful transfer of a data frame shown in FIG. 2), data transfer is requested, and waiting for the reception of an ACK frame is implemented (step S20).
However, a wireless link state of a preset route is unstable, and therefore, a data frame cannot be delivered up to the data transmit/receive apparatus of the destination. Accordingly, the data transmit/receive apparatus of the departure location cannot receive the ACK frame. Therefore, the data transmit/receive apparatus of the departure location fails to receive the ACK frame until a timer is terminated, and repeatedly performs steps S11 to S13 for a predetermined number of attempts.
The data transmit/receive apparatus of the departure location repeats this process up to three times, and if the data transmit/receive apparatus of the departure location cannot receive a special ACK frame, the apparatus of the departure location does not attempt to transmit the data frame again, but instead confirms that the transfer of the data frame has failed (step S45).
Case 3. The Transfer of a Data Frame has been Successful but the Transfer of an ACK Frame Fails
With reference to FIG. 4, in a case where the transfer of an ACK frame has failed, as in the case described above (i.e., in the case of successful transfer of a data frame), data transfer is requested by performing steps S11 to S15, and the apparatus at the departure location waits for receipt of an ACK frame as implemented in step S20. In addition, in FIG. 4 the ACK frame is transmitted through steps S31 and S32, but fails in transit and does not reach the original sending apparatus.
However, since a wireless link state of a preset route is unstable, the ACK frame transmitted from the data transmit/receive apparatus of the destination cannot be delivered up to the data transmit/receive apparatus of the departure location.
Finally, the data transmit/receive apparatus of the departure location fails to receive the ACK frame within the time counted the timer expires. Accordingly, the data transmit/receive apparatus of the departure location, and a data transmit/receive apparatus located on the transfer route repeatedly performs steps S11 to S15. The data transmit/receive apparatus of the departure location repeats this process up to three times, and if it cannot receive a special ACK frame, it does not attempt to transmit the data frame again, but finally confirms that the transfer of the data frame has failed (step S45).
In order to embody a WSN, the reliability of each data frame transmitted among nodes must be secured. However, in the WSN based on the protocol that the prior IEEE 802.15.4 standard has proposed, as described above, nothing is done, but the re-transfer of a data frame is performed, and an alternative pertinent response to a failure of re-transfer has not been proposed.